How do you know when baby is done?

Ch
My daughter is 3 mons now. Everyone always told me you will know when the baby is done nursing by looking at their cues, or theyll simply pull off the breast when done...mine does neither of those things! Hasnt for 3 months. I know shes getting plenty & growing just fine, but if possible id like to shorten her feeding....she will stay on the boob all day if i let her! Anyone have any suggestions?